lhc detectors (atlas) shlomit tarem technion, israel inst. of tech
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LHC Detectors (ATLAS)
Shlomit TaremTechnion, Israel Inst. of Tech.
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The LHC and its detectors
The LHC, a pp collider with 14 TeV pp cm energy will start operation in 2008
4 experiments are working to finish assembly and commissioning ATLAS – general purpose – discovery of new particles CMS – general purpose – discovery of new particles LHCB – B Physics – forward ALICE – heavy ion physics
LHC collisions are a difficult experimental ground We won’t know the cm energy of each collision There will be many pp collisions on top of each other Most of the collisions are due to uninteresting physics There will be too much data to collect
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LHC Design Parameters
Energy at collision 14 TeV
Luminosity 1034/cm²/s
Bunch spacing 7.48 m
25 ns
Particles/bunch 1011
Collisions per BC 23
Luminosity lifetime 10 h
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ATLAS and CMS will start operation at the LHC at the end of 2007
Higgs bosons or alternatives for SSB
CP-violation with high precision
Rare B decays Top mass SUSY particles? Beyond the SM
The ATLAS and CMS Experiments
The ATLAS detector
First conclusive Higgs search
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Particle detection basics
Fast particles created in LHC collisions will interact with the detector in various ways and leave signals in it Charged particles will ionize it Electrons will radiate in it Photons will produce e+e- pairs Hadrons will interact with nuclei
We use these interactions to build detectors The different interaction of different particle types
with the detector help us distinguish between them Different technologies help distinguish between
different particle types Stable particle types which leave signals in the
detector include , e, , , k, p, n and hypothetical exotics
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A modern detector is like an onion The collision point is surrounded by a magnetic
field to bend charged particles according to their momentum In the field region is a tracking detector to measure particle
trajectories and bending Next are Electromagnetic calorimeters which utilize EM
showers to stop electrons and photons and measure/sample their energy
Then there are Hadronic Calorimeters which utilize nuclear interactions with detector material to create and measure hadron showers and stop hadrons
Outside are muon detectors – another tracking detector for the only known charged particle type which is not stopped in the calorimeter
The muon detector may have it’s own magnets – then it’s a muon spectrometer
ATLAS has such a magnet for muons CMS has all detectors inside one big magnet
Particle detection basics
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Ionization energy loss Relativistic particles lose energy by ionizing atoms of
the material they pass Ionization occurs randomly at points along the particle path We detect the ionization positions to find the particle
trajectory The amount of energy loss per unit path length, dE/dx,
depends on the particle charge and velocity and atomic properties of the medium
For a known medium, and since most stable particles have 0 or unit charge, dE/dx is a tool to find the particle velocity
Knowing the momentum and velocity we can obtain the particle mass
Tracking detectors aredesigned to measure energyloss positions
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Electromagnetic showers Relativistic electrons lose energy primarily via
Bremsstrahlung radiation due to acceleration by multiple scattering Energy loss by Brem is proportional to E Energy loss by ionization is proportional to ln(E)
Photons create e+e- pairs The distance over which these
happen is characterized by a “radiation length” A characteristic of the medium The distance over which an
electron is left with 1/e of it’s energy The average path length for pair
creation The repeated occurrence of
Brem and pair production create an EM shower
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EM showers
The number of particles at each stage is N(t)=2t
The energy per particle is E(t)=E02-t
The process continues until the electrons go below the Brem threshold Ec
The total number of electrons in a shower is proportional to the initial particle energy
EM showers are narrow and well contained A shower of a 100 GeV
electron in lead is 4 cm wide and 16 cm long
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EM calorimeters
A calorimeter creates a shower and measures the number of secondary electrons produced in it
A radiator is a heavy material with short radiation length, which advances the shower process
Between radiators we place measurement layers to measure how many electrons pass each layer
The measurement is either via ionization energy loss or via scintillation
Some materials can radiate and measure (lead glass) To measure correctly the electron/photon energy the
calorimeter has to be deep enough to stop the whole shower
Muons and hadrons leave an ionization trail in the EM calo
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Hadronic showers
Hadron have strong interactions with the detector nuclei New particles, mostly pions, are produced and continue to
interact The differences from EM showers:
Greater distance between collisions More than 2 particles produced per
interaction Particles stopped at ~200 MeV Larger scattering angles – wider
shower If a 0 is produced it’ll start an EM shower Large statistical differences in measured energy between
showers from similar particles This is the only way to detect neutral hadrons Both EM and hadronic showers are detected via ionization
losses of the resulting particles The EM calorimeter is the first layer of the Hadronic
calorimeter
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Other interactions with matter
Scintillation In some materials 1-3% of the ionization e-loss goes into
optic or near optic photons The light can be collected – very fast detectors Used in the ATLAS tile calorimeter
Cerenkov radiation Radiation created when the passing
particle is faster than the speed of light in the medium
Can help distinguish between particle types in energy ranges depending on radiator
Transition radiation Radiation induced when a particle passes between two
media Also used to distinguish between particle types Used in ATLAS tracking
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Reconstructing an energetic collision
In order to understand a collision we need to know When did the collision happen The directions of final state particles
Ionization trajectories of charged particles Shower position center for neutrals
The momenta and energy of final state particles Charged particle momenta from bending in B field Neutral energy from EM or hadronic energy deposition Velocity from TOF, dE/dx or Cerenkov angle Energy and momenta of unstable particles from conservation
laws What type of particle?
Specific interaction – EM shower for electrons, lack of it for muons
Mass calculation from momentum and velocity Particle spin? From decay angular distribution Lifetime? Secondary vertex and proper decay time
reconstruction
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Important detector characteristics Time resolution t Spatial resolution x Energy resolution E Detection efficiency Misidentification probability Two track resolution x
Detector characteristics derived from the above
Momentum resolution from x and the B field integral Velocity measurement resolution
From t if by TOF From E if by dE/dx From x if by Cerenkov
Cost, stability (aging) and longevity are also important for detectors
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Gas wire chambers
Detection of ionization on a particle trajectory by electrons drifting to a wire at high potential is known since Rutherford built a gas tube with a central wire in 1900
At a high potentials the drifting electrons are accelerated and ionize additional atoms in their path
An avalanche is formed, creating amplification >105
In MWPCs (G. Charpak, Nobel prize 1992) a plane of anode wires at high potential is arranged between two cathodes with amplifying gas between.
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MWPC and TGC
A particle passing in gas will leave a trail of electron clusters (+ ionized gas atoms). The electrons will drift in the E field towards the closest wire, and will create an avalanche and charge on the wire. The charge is read by readout electronics.
Since the signal arrives from the closest wire to the particle passage, the “hit” resolution is the distance between wires.
Parallel to the wire direction the position can be obtained by Charge division between the wire ends (resolution 1% of wire length) Difference in time of arrival on the 2 sides (resolution ~3 cm) Measuring the induced charge on pick-up strips on the cathode
(resolution 30-100 m) With the last method there may be
ambiguities In ATLAS the end-cap muon trigger (TGC) is made this way
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Drift chambers
In drift chambers we measure the time between the passage of the energetic particle and the signal arrival to the wire.
This allows to estimate the distance from the wire where the cluster was produced, providing an accurate hit position measurement
The electron drift velocities is ~50 m/ns with little dependence on the field – the position resolution is 50-200 m
Traditionally large drift chambers surrounded the IP, now largely replaced by semi-conductor trackers
The ATLAS Monitored Drift Tube (MDT), the precision muon chambers, are a kind of drift chamber
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Semiconductor trackers
Charged particles produce electron-hole pairs in O(nm) thin reverse bias junctions – ionization again The high electron density and low ionization potential (3 eV
compared to 30 in gas) result in large signals in thin sensors without the need for multiplication
The electrons/holes are collected on electrodes subdivided in thin micro-strips or pixels of 20-100 m
The detectors are fast because of the short distances The charge is collected via tiny bump bonds connected to
the readout electronics
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Basic particle identification
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Advanced particle identification
dE/dx
Threshold CerenkovRing Imaging Cherenkov
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Comments on measurement accuracy Measuring a charged particle trajectory in a magnetic
field is an accurate way to measure momentum and direction Charged particles are easier to detect accurately This affects which decay channels to measure
At low energy, an EM calorimeter is less accurate At high energies the EM calorimeter is competitive, but
since it’s far from the interaction point, the creation vertex of the particle is unknown
Semiconductor trackers are very accurate but expensive Readout is an issue, especially for pixel detectors Many dense readout channels required Very fine connection of readout to sensor - difficult
Silicon detectors used together with coarser measurements In ATLAS with transition radiation tracker
EEE %10
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Reconstructing short lived particles Reconstruct from decay products
Identify possible decay products Calculate invariant mass Reconstruct secondary decay vertex
Need decay channels with easily identified final state
Background Combinatorial background from unrelated tracks falling
randomly in the mass window Particle misidentification (fake muons or electrons) Misaligned detector causes widening of invariant mass –
more background
Secondary vertex reconstruction for Ks, , b-hadrons
Impact parameter cuts in reconstruction may reduce efficiency for Ks,
b from D0
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The ATLAS detector
η
We work in the coordinate system , , z
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Inner Detector
The ATLAS Inner Detector (ID) is inside a 2T solenoid magnet
There are 3 detector types: semi-conductor pixel semi-conductor strips transition radiation
tracker The pixel and SCT will
provide a few very accurate points
The TRT will providecontinuous tracking –36 points
Each contributes similarlyto the resolution
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Pixel detector
3 barrel and 8 disk layers of 140 MILLION pixels on 2228 Silicon semiconductor modules
The 140 MILLION channels are read out providing a resolution of 10 in r- and 50 in z
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SCT
SCT is designed to provide eight precision measurements per track in the intermediate radial range
contributing to measurement of Momentum Impact parameter Vertex position
In the barrel SCT eight layers of silicon microstrip detectors
The end-cap modules use tapered strips with one set aligned radially.
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TRT
The Transition radiation Tracker is based on the use of straw drift detectors – like miniature MDTs can operate at high rates due to their small diameter
and the isolation of the sense wires within individual gas volumes
Electron identification capability is added by employing Xenon gas to detect transition radiation photons created in a radiator between the straws.
Each straw is 4 mm in diameter and equipped with a 30 µm diameter gold-plated W-Re wire The barrel has ~50,000 straws The endcaps have 320 000 straws
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Calorimeter
The EM calorimeter, and part of the Hadron calorimeter are made of an accordion like arrangement of lead radiator and liquid argon measurement medium
There are over 100000 channels in the barrel and 70000 in the endcap
The calorimeter takes part in the level 1 trigger
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H Why is this channel so difficult? The final state is 2 neutral particles
No momentum and direction measurements in the tracking detector are available
Photons shower in the EM calo, with energy resolution
The invariant mass of a pair of photons has to be calculated – mass resolution is related to the single particle momentum resolution
We expect a wide distribution Almost every 0 decays into 2 photons
There are many 0 produced in each collision Highly boosted 0 produce very close to each other The calorimeter has to be highly segmented to tell one from 2 0 is a big combinatorial background under the H peak.
This channel dictated the design of the EM calo
EEE %10
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H
Signal and background After background subtraction
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Tile hadronic calorimeter
In the central region <1.3 there is also a scintillating tile hadronic calorimeter Steel is the absorber material (radiator) causing showers Particle showers are sampled by tiles of scintillating
plastic which emit light when charged particles go through them.
The light pulses are carried by wavelength shifting optical fibers and converted to electronic signals
The tile calorimeter is highly segmented 0.1x0.1 in , 3 radial segments
Can help identifying Narrow (ionization)
signal continuinginto the outer layer
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Jet energy scale
The signal in the calorimeter requires translation into the energy of the particle
This translation is particle type and detector region dependent Pions leave a different signal than electrons for the same
energy loss Different sampling depths result in different calibration
Jets are more complicated still 0 and charged , but also muons/electrons/neutrino
These calibrations are started at test-beams Continue using simulation Will continue using well understood samples
Z+jets
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“Missing energy”
The total cm energy will be 14 TeV Most final state energy will go down the beam-pipe
unmeasured Hard interaction energy unknown and differs by
event Products characterized by momentum transverse to the
beam-line pT
No way to measure “missing energy” out of unknown total
What we measure is the pT imbalance in the final state
sddsZs
uuR
01
01
01
~~~
~~
eeZ 0
Measured as vector sum of energy deposition in calo cells
Characterizes events with particles that leave the detector unobserved
No missing ET Missing ET
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Missing ET continued
What particles result in missing ET? Neutrinos The SUSY LSP or neutral stable NLSP Muons?
They leave little energy in the calorimeter, so if not accounted for, will produce fake missing ET
They are not accounted for in the calorimeter trigger so high pT muons can produce a missing ET trigger
This should be corrected at Event Filter or offline Charged stable NLSP?
Like muons No other source of missing ET in event
Detector malfunction can fake missing ET
A “hot” or “dead” area in the calorimeter willchange the ET balance artificially
Particles going through cracks also createfake missing ET
Fake missing ET
eeH
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Missing ET resolution
A lot of work on understanding missing ET and its dependence on topologies jet energy calibration
e// energy corrections crack and dead areas Jet punch through seen as muon
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The ATLAS detector – The Muon spectrometer
Trigger chambers
Precision chambers
Trigger chambers RPC and TGC are
used for triggering, measure 2 coordinates, and
Precision chambers The MDT are used
for precision measurement and measure only
The CSC measures precisely and coarsely
Tracking requires combining the information from all sub-detectors
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Monitored Drift Tube chambers
Precision measurements in the muon spectrometer are performed by chambers of Monitored Drift Tubes (MDT) The basic elements are aluminum tubes with a 3 cm
diameter and a wire at HV in the middle The basic measurement is the drift time of ionized
electrons to the wire The measurement resolution is ~80 m Each chamber has 2 superlayers, each with 3 or 4 layers
of tubes
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The radius from which the electrons drift to the wire is calculated from the time measurement
These R-T relations have to be calibrated constantly to maintain the resolution
The segment is tangent to the radii
To maintain resolution we also need to know exactly where each tube is alignment is a big issue
Hit radius reconstruction in the MDT
t0t=t0+tdrift
R=R(t-t0) =R(tdrift)
tdrift
Segment reconstruction in MDT
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Reconstructing muons in ATLAS
Muons appear in many heavy particle decays this makes them interesting
They are by far the easiest to identify Just look for energetic particles outside the calorimeter
Their momentum may be measured in the muon spectrometer outside of the mess of tracks in the inner detector
The experiment output is a list of hitchannels and some information on thehit For MDT – drift time For trigger chambers – Beam Crossing ID For CSC – pulse height distribution
Noise hits too…(MDT)
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MDT RPC/TGC
Muon reconstruction in ATLAS detector
End Cap
Toroid
Barrel Toroid
Calorimeter
Inner
Detector
MDT RPC/TGC
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Muon reconstruction in ATLAS detector
Track reconstruction in the Muon Spectrometer is done with MOORE or MuonBoy
Large volume toroidal field – bending in η direction
Low detector occupancy Accurate high momentum
measurements
++ ++
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In ATLAS, muon tracks can be reconstructed independently in the muon spectrometer. A search for all is performed
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Muon reconstruction
Short segments of the trajectory are found is the MS stations
The segments are then connected into tracks We know the B field
and thus the trajectory of a of a given momentum
The momentum isobtained from the track fit
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Muon reconstruction in ATLAS detector
Similar programs reconstruct tracks in the Inner Detector
Reconstruction of all charged particles is done in the Inner Detector
High track multiplicity Bending in φ direction
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Muon reconstruction in ATLAS detector
Following this, muon tracks or segments are combined with inner detector tracks to obtain the muon momentum at the interaction point
MuId/Staco Extrapolate muon tracks
back to the primary vertex region
Combines them with Inner detector tracks
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Muon Reconstruction
A different program, MuGirl, identifies muons by associating muon hits and segments to an inner detector track in order to flag the track as a muon[1] Initialize Muon candidate from ID track parameters [2] Extrapolate track to Muon Spectrometer chambers [3] Look for hits in a road around the track extrapolation [4] Make segments from hits [5] Improve extrapolation by
using segment information [6] Collect hit & segment
information to identify muon[8] Select “muon like” candidates
This method works betterfor low pT muons
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H4, 22e
Best channels for finding the Higgs Good trigger with high pT muons
Low pT muon reconstruction an issue for low mass Higgs Lowest pT muon under 10 GeV for many events
Could require 2 high pT muons w Z mass and collect additional ones
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Triggering at the LHC
Object What physics?
eHiggs, new gauge bosons, extra dimensions, SUSY, W, top, B-physics,
Higgs, extra dimensions, SUSY, B-physics
Higgs, new gauge bosons, extra dimensions, SUSY, W, top, B-physics
Jets SUSY, compositeness, resonances, B-physics
The LHC event rate is too high to collect
Selection of physics signals by identification of objects that can be isolated from the high particle density environment.
Event rateEvent rate
Level-2Level-2
Level-1Level-1
Offline AnalysesOffline Analyses
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The ATLAS Trigger
The 3-level trigger selects interesting events at an output rate of 100 Hz from the input rate of 40 MHz The Level-1 (LVL1) trigger – 40 MHz to 75 KHz
Uses custom electronics to make the decision in hardware Uses low granularity data from a subset of trigger detectors Identifies Regions of Interest Identifies bunch crossing of interest Has 2 sec to complete each selection
The Level-2 (LVL2) trigger – 75 KHz to 5-10 KHz Uses the full granularity data Starts from Regions of Interests flagged by LVL1 Only data requested by the algorithms are read out. The average time budget ~10 ms.
The Event Filter (EF) – 10 KHz to 100 Hz Uses complete event information Time budget of a few seconds. Accepted events are written to mass storage
hard
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eso
ftw
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e
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Goal of the level 1 muon trigger
Select from b, t, W, Z, H Low pT for b pT>6 GeV
High pT for Higgs pT>20 GeV
Look for muons from the interaction point Eliminate cavern
background Eliminate beam halo and
cosmic muons Reduce background
from decay in flight of /K
pT of muons from different processes
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Trigger scheme
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In passing the b field Awill bend up Awill bend down The window between
them contains all with pT>threshold
For large pT the window becomes small, and we need a longer lever-arm to resolve it – add another station
Endcap muon trigger – more detail
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Windows
pT thresholds are determined from the maximal acceptable rate Each trigger type gets a
bandwidth Flexibility is required
Window sizes for each pT/η/φ are found from simulation
The actual selection is done in hardware
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The charge created in the chamber is digitized by an ASD
The digital signal passes in cables 2-10 meters long
They are received at the trigger electronics PS-Pack ladder on the TGC sector
Endcap muon trigger – the electronic implementation
There is 1 PS-Pack ladder for each 1/24 triplet and doublet-pair
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electronic path scheme
triplet
triplet
innerdoublet
innerdoublet
pivot doublet
pivot doublet
wiretriplet
Slave boards
striptriplet
wire
wire doublet
stripdoublet
strip
High pT
boardswire
strip
sector
logic
pp
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Level-1: Calorimeter
Calorimeter Trigger looking for e/ + Jets + t
objects Using trigger towers of
Hadronic and Electromagnetic calorimeters
The requirement for a trigger object: The RoI cluster is a local
maximum The most energetic cluster > ET
Total ET in EM isolation < EM Isolation Threshold
Total ET in Hadron < Hadronic isolation threshold
Example of e/ trigger algorithm:
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Missing ET trigger
At level 1 – jet energy sum processor computes total scalar ET, Ex and Ey
Missing ET not an inclusive trigger but combined with single jet or electron/photon or hadron/ triggers which may not pass level 1 by themselves
Envisioned missing ET thresholds could start ~70 GeV Does not fit the RoI mechanism – global by definition At level 2 unpacking the data from 200,000
calorimeter cells is prohibitive corrections for known level1 deficiencies calculating missing ET from jet RoIs
It may be too slow even for the EF, in this case the missing ET may be calculated from jets rather than calo cells
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The CMS trigger
CMS has a 2 level trigger LVL1
Uses muon chambers and calorimeter Finds e, jet, candidates above thresholds 40 MHz 100 KHz
HLT Uses algorithms similar to offline 100 KHz 100 Hz Inclusive b,c, trigger (high pT jet) Partial reconstruction of exclusive decays around μ ROI
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The ROI mechanism
At level 2 the processors run algorithms seeded by level 1 Regions of Interest (RoI)
For each RoI the algorithm fetches the relevant data from subdetectors which did not participate in the level 1 decision
Level 2 algorithms are run in a sequence, refining the decision in stages
They create new seeds for the Event Filter
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02
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1 in 5 000 000 events is kept
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Trigger issues for b-physics performance
LHC is geared towards “Discovery Physics” B physics is a side show
B-physics performance is impacted strongly by trigger menus Characteristic B-physics triggers are at low pT
The experiments have multilevel triggers Level-1 is in hardware – designed for 40MHz100KHz The level-1 trigger for B-physics is based on one or more muons
Acceptable trigger rates in ATLAS and CMS have been reduced due to “staging” of high level trigger processing power Envisioned trigger menus include 2 low pT or one higher pT Algorithms are developed for recovery of events at level-2
First luminosity is expected to be lower and that will enable collecting 6 GeV single muons at the beginning
Detector calibration depends on channels that are also good for B Physics - J/ and
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Algorithms to recover events at level-2
The level 1 trigger output of is ~20 KHz of events with at least one muon with pT > 6 GeV 4 KHz from b events Most triggers from cavern background or muons from K/
decays, At the level 2 trigger this rate must be reduced by x100
This may be achieved by confirming a muon in the Inner Detector in addition to confirming it in the Muon Spectrometer
Then cutting harder on pT
This selection criterion removes many interesting b events We would like to achieve
higher efficiency for the “gold” channels (J/) at level 2 After
Level-2
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Example of level 2 algorithm
The rate of J/ and +– events is low enough for the second level trigger
A di-muon trigger will allow an effective selection of channels with J/ +– and rare di– b decays
One way is dimuon trigger at level 2 based on a single muon trigger at level1
The second muon, usually lower pT, is found by searching in an extended region of interest around the level 1 RoI
single-muon
di-muon
all
all
h
h
b
b
c
c
J/
@1033cm-2s-1
Cro
ss s
ecti
on
, (n
b)
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μμ
RoI ( φ, η )
Create the pair of tracks with
opposite charge
Dimuonrecovery at level 2
MDT RPC/TGCLVL1 pT() > 6GeV
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Results from this algorithm
level 1 muon RoI
Enlarged muon RoI
level 1 muon RoI
Enlarged muon RoI
J/ψμ(pT>6GeV)μ(pT>3GeV)J/ψμ(pT>2.5GeV)μ(pT>4GeV)
J/ψμ(pT>6GeV)μ(pT>3GeV)J/ψμ(pT>2.5GeV)μ(pT>4GeV)
J/ψμ(pT>6GeV)μ(pT>3GeV)J/ψμ(pT>2.5GeV)μ(pT>4GeV)
The efficiency of J/ψ identification vs. pT of the lower pT muon
Efficiency of J/ψ (relative to level 1) vs. fake rate for different cuts
The efficiency to find J/ψ vs. the size of window opened around the level 1 μ RoI
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Detector issues for new Physics The case of a new long lived particle
Heavy charged long lived particles exist in many theories beyond the standard model A case in point is GMSB where the stau is the NLSP and
couples weakly to the gravitino. The signal we look for is a charged particle with low
hence referred to as stau Any slepton would have the same signature R Hadrons also have strong interactions
An existing lower limit gives the stau M>100 GeV/c2
Imagine a 100 GeV/c2 stable charged particle going through a detector with pT of 100 GeV/c
This cannon-ball should be easily visible – we can’t miss it…
Think again!!
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How would the look in ATLAS
A very slow stau would lose a lot of energy by ionization A 100 GeV/c2 stau with pT < 25 GeV at eta=0.1, would be
absorbed in the calorimeter. Likewise, a 200 GeV/c2 central stau with pT < 35 GeV
BUT A particle with >0.5 would lose less than 7 GeV A particle with >0.8 is almost minimum ionizing
Particles with <0.6-0.7 will arrive in the muon spectrometer with a different beam crossing
Signals in the ID and Muon Spectrometer may be modified due to higher ionization
~
The following study was done for a stau with a mass of 100GeV/c2 as introduced in GMSB point 1 of
CERN-TH/2000-206
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ATLAS length > 20m & Collision period = 25 ns 3 events coexist in the detector at the same time
To match correctly event fragments from different sub-detectors BCID is crucial
BCID is based on time measurements, each detector unit is calibrated with respect to particles which move almost at the speed of light ( =1)
(stau)<1 so it may be marked with the wrong BCID
Timing issues for a heavy charged particle
Delay in arriving to the muon spectrometer wrt a muon in units of BC
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The stau data is associated with event N+2
and LVL1 - the case of a “normal trigger”
Assuming a non stau trigger on event number N
A with pT > 75 GeV can give a missing ET trigger The resulting readout requirements are the same as above
The stau data is associated with event N+1
Muon trigger chambers (TGC and RPC) should read out BCs N, N+1, N+2.
The MDT always reads out many BCs.
~
The case of a calorimeter trigger~
calorimeters
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The muons were here in event N-2
and LVL1 - The case of a muon stau trigger
A with pT > 30 GeV can give a high pT muon trigger. Assuming the triggered event number N.
All sub detectors have to read events number N, N-1, N-2
All particles were here in event N-2
~
~~
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Different trigger scenarios result in different readout requirements
The different sub-detectors have the ability to acquire data from different (more than one) BCs.
BUT
Readout programming can not be changed by trigger type.
Moreover, it can not be changed during ATLAS run time
the decision of which events are to be read by each sub detector will have a dramatic effect on ATLAS’s ability to discover the
and LVL1 - conclusion~
~
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Possible data taking mode
Muon spectrometer collects data from events N, N+1 and N+2 Inner Detector collects data from events N, N-1 and N-2 Calorimeter collects data from events N, (N-1 and N+1)
Lost Data
If the stau produced a muon trigger, and there was also a muon in the event (that didn’t trigger), then the muon spectrometer data related to that muon is lost
Open questions Is it possible to acquire data from more events at all levels? What needs to be done to actually do it? How does this data taking mode effect the data size ?
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Identification of the in RPC The RPC chambers have great time resolution -
3.125ns The BC and the time within the BC are known it is
possible to calculate the Time Of Flight (TOF) from the interaction point
Apply the TOF calculation to the barrel LVL2 algorithm muFast to get initial estimation of the particle’s speed
~
Estimation in muFast for Different generated
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The RPC TOF can be estimated at the level 2 trigger
An event identified at LVL2 as containing a slow high pT particle could be moved directly to a rapid analysis track
reject ~80% of the muons
reject ~97% of the muons
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Hit radius reconstruction in the MDT
The long time window of the MDT guarantees that data of low particles will be saved.
The measured hit radius is incorrect
The segment is tangent to the radii Larger radii result in
Badly fitted segment Wrong direction segment
t0t=t0+tdrift
R=R(t-t0) =R(tdrift)
t0+ttstau=t0+t+tdrift
Rstau=R(tstau-t0) = R(tdrift+t)>R
tdrift
Segment reconstruction in the MDT
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A reconstruction algorithm Relies on long time window of MDT and BCID from ID Identify penetrating particle by associating muon hits and
segments with extrapolated ID track Loop over possible t0s
Change MDT digits’ time and hence radii. Create MDT segments from the re-timed digits.
Choose the segment with the best 2. Obtain the real t0 (TOF) as the one
that minimizes the 2
Calculate
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GMSB – Example points Background
Main background is from muons with pT>40
(>40)/(stau point 1) ~ 25
distribution not from model
Mass reconstruction
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Offline analysis – Signal and Background Preliminary Results Minimal cuts
<0.99 Reasonable 2 Segments in all the 3 stations
Background will be reduced by better reconstruction
No cuts With cuts
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Heavy charged particle summary
If nature cooperates, we have a chance to find such a particle
However, this requires paying attention to details of detector and trigger operation
Some modifications are needed to previously envisioned operation
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Summary
We expect/hope the LHC will be an exciting place to do physics – the new energy gives space for discoveries
Detector knowledge was required to design a detector (two) which can find the interesting physics
Understanding the detector will help us in our analysis
Theorists should understand what measurements are more/less possible as a guide to choosing the channels they calculate
Theorist could use this info to understand how to assess experimental measurements